This invention relates to a concept of manufacturing components using numerically-controlled machine tools, and relates to systems for machining components and including workpiece-handling equipment and equipment for controlling the detail and overall operation of the system.
Despite recent developments in forging, extrusion, powder metallurgy, electro-chemistry, high-energy-rate forming and other methods, the most important engineering processes involved in the manufacture of components in small batches of, say, 100 or less, are metal cutting processes, e.g. milling, drilling, turning, or other conventional ways of physically removing unwanted material.
Batch production is generally accomplished today by issuing components into manufacture on an "operation" basis, i.e. the work to be done is split down into separate operations, sometimes involving as many as twenty operations for one part. Each of these may involve transferring the part from one machine tool or process to another and then to a third and fourth and so on, but even when many of the operations are confined to one machine tool, changes in the set-up or position of the workpiece can be frequent. These changes in set-up result in the machine tool not cutting during the change-over period so that the ratio of cutting time to total working time is of the order of 15-20%. Where the component moves from machine to machine the situation may be far worse. Even with good organisation it is rarely possible to manage a large engineering shop so that components spend less than a day between operations. Frequently this period may be nearer to one week. Even with only a few operations, the queuing problems associated with machine loading lead to a total manufacturing cycle which is rarely less than three months and is frequently as much as six months.
The number of components forming a workshop float or work in progress may be extremely high, in some cases as high as one million, and this may be the minimum necessary with current production control methods to maintain stability and give good machine loading. Such work-in-progress represents a large investment in partly finished material as well as in loss of lead time. An extremely complex and expensive production control system is required to progress the work from operation to operation and, although a computer can be used to improve the situation, such measures are no more than a palliative which does little to remove the main disadvantages of the system.
In relatively recent times it has become common in some industries to employ so-called "transfer machines" for large-scale production of machine components, For example, such machines are used for carrying out the necessary machining operations on cylinder blocks for automobile engines. Such machines are arranged with a number of positions termed "stations" to which each component is brought successively by a conveyor and at each station one or more predetermined operations is performed, e.g. milling or grinding exterior surfaces to which other parts must be accurately fitted, drilling oilways, reaming and honing cylinder bores, the components being located at each station by clamps engaging datum surfaces of the components. These transfer machines are however essentially mass-production machines; their initial cost is high, and they are inflexible in operation, as considerable time and expense is involved in changing the set-up of such a machine to vary its fixed programme of operations to allow for a change in the components to be produced; in most if not all cases, the expense of a change is such that, each time the machine is set up for a particular component, there must be a production run of months or even years if operation of the machine is to be economically justifiable.
Also in relatively recent times, complex numerically-controlled machine tools, known as "machining centres", capable of performing different types of machining operations on a workpiece at one setting, and sometimes pallet loaded, have become available. In such machining centres, sacrifices usually have to be made in machining speeds and rigidity in order to achieve the complex movements. Also, such complex machine tools cannot readily be made in twin spindle form, which involves a sacrifice of a factor of 2 in effective machining speed. Further, although a large number of components may require complex machining facilities, in probably only a relatively small proportion of these components will this sort of machining amount to more than a small proportion of the total machining required on that component; thus the single complex machine will spend a large proportion of its time carrying out relatively simple machining operations, so that its capacity for doing complex work is wasted to a great extent.